1. Field of the Invention
The present invention relates to a circuit for generating a very small electric current.
2. Description of the Related Art
A conventional CR oscillation circuit and load driving circuit with an over-current protection function as shown in FIG. 9 determines a discharge time period on the basis of a very small electric current. As shown in FIG. 9, a pnp transistor 100 is connected in series with a resistor 101 and constant current circuit 102 between a voltage source and ground, while the base of the pnp transistor 100 is connected with the connecting point between the resistor 101 and constant current circuit 102. The above-mentioned series circuit allows a constant current I1 to flow. Further, a pnp transistor 103 is provided between the voltage source and ground, while the base of the pnp transistor 103 is connected with the collector of the pnp transistor 100. Thus, the pnp transistor 103 also allows a very small electric current Ic proportional to I1 to flow.
The CR discharge oscillation circuit as shown in FIG. 10 can be constructed by utilizing the above-mentioned circuit as shown in FIG. 9. The collector (through which Ic flows) of the pnp transistor 103 is connected with transistor 104 which is a part of a current mirror circuit comprises another transistor 105 which is connected with a capacitor 106, thereby constructing through a switch 107 a charging circuit 108. Thus, the discharge current of the capacitor 106 is determined by Ic.
Here, Ic is determined by the following formula, if the transistor 100 has the same characteristics as the transistor 103.
Ic=I1/exp((I1R1q/kT)),
where R1 is a resistance value of the resistor 101, q is the unit charge of electron, k is Boltzmann constant and T is absolute temperature.
Therefore, it is difficult to stabilize Ic within a whole defined temperature range, because Ic is strongly dependent upon the temperature.
Accordingly, the discharge time period becomes shorter at a higher temperature than at a lower temperature. If the discharge time period is designed for a higher temperature, then the lower temperature discharge time period becomes unduly long.
An object of the invention is to suppress a fluctuation in the very small electric current (in a small current generating circuit such as a CR oscillation circuit and load driving circuit with an over-current protection function) due to temperature fluctuation, thereby stabilizing the very small current.
The present invention includes several Features stated below.
According to Feature 1, the very small current generating circuit comprises:
a first current route for a first current wherein between an internal reference voltage terminal and a ground terminal, a first resistor 10 is connected in series with a first npn transistor 11 of which collector is connected with said first resistor 10 and of which base is connected with said collector;
a second current route for a second current of a negative temperature characteristics and proportional to said first current wherein between an external voltage source and said ground terminal, a first pnp transistor 12, a second resistor 13, a second npn transistor 14 and a third resistor 15 are connected in series in this order, said second resistor 13 being connected with the collector of said first pnp transistor 12, said third resistor 15 being connected with the emitter of said second npn transistor 14, the collector of said second npn transistor 14 being connected with said second resistor 13, the base of said first pnp transistor 12 being connected with the collector of said second npn transistor 14, the base of said second npn transistor 14 being connected with the collector of said first npn transistor 11; and
a third current route for a third current proportional to said second current which is outputted as a very small current wherein between said external voltage source and ground terminal, connected is a second pnp transistor 17 of which base is connected with the collector of said first pnp transistor 12.
Thus, the very small current is stabilized inspite of the temperature fluctuation.
According to Feature 2, the very small current generating circuit further comprises a fourth resistor 16 which is connected between said external voltage source and the emitter of said second pnp transistor 17.
Thus, the current through the third current route is adjusted by the fourth resistor 16.
According to Feature 3, the very small current generating circuit further comprises a fifth resistor 18 which is connected with the emitter of said first npn transistor 11, wherein a temperature coefficient of said fifth resistor 18 is smaller than that of said third resistor 15.
Thus, the slope of the temperature characteristics of the current through the third current route is further adjusted.
According to Feature 4, the very small current generating circuit further comprises a fifth resistor 18 which is connected with the emitter of said first npn transistor 11, wherein a temperature coefficient of said fifth resistor 18 is negative and a resistance value of said third resistor 15 is negligibly smaller than that of said fifth resistor 18.
Thus, the slope of the temperature characteristics of the current through the third current route is further adjusted.
According to Feature 5, in the very small current generating circuit, said internal reference voltage is generated on the basis of a voltage of said external voltage source.
Thus, the current through the third current route is further stabilized due to stabilized internal and external voltages.
According to Feature 6, in the very small current generating circuit, said second resistor 13 comprises a plurality of resistors 30 and 31 with different temperature coefficients.
Thus, the current through the third current route is finely adjusted and less temperature insensitive.
According to Feature 7, the very small current generating circuit further comprises:
a transistor 28 constructing a current mirror circuit;
a transistor 29 constructing together with said transistor 28 said current mirror circuit and provided at an output terminal for outputting said very small current in said third current route;
a capacitor 26 connected with said transistor 28;
a charging circuit connected with said capacitor 26 through a switch 25 for charging said capacitor 26.
Thus, an accurate CR oscillation circuit is constructed due to the stabilized very small current (the current through the third current route).
According to Feature 8, the very small current generating circuit further comprises:
an over-current detecting circuit 23 for switching on said switch 25, when a current through a load 21 becomes greater than a prescribed current;
a comparator 27 for comparing a voltage of said capacitor 26 with a threshold voltage and outputting a signal for switching off a power transistor 20 on the basis of the comparison result.
Thus, a load driving circuit with an over-current protection function is constructed due to the stabilized very small current (the current through the third current route).
According to Feature 9, in the very small current generating circuit, said threshold voltage comprises a lower threshold voltage and a higher threshold voltage.
Thus, the discharge current is stabilized and held within a prescribed range.
Further, the very small current generating circuit of the present invention can be applied to a timer circuit and filter circuit.